Load and store external references using the root register rather than
generating a constant and dereferencing it. This typically uses 1
instruction rather than up to 4.

Also adds external reference store optimisation for arm64.

Bug: v8:7844
Change-Id: I5f73728e7a72e366a31bfb694581e2e7d8250947
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2007270
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65950}